What type of creatures are the young ladies of Mrs. Poole?

According to the Penny Dreadful Wikia... yes, there is one... She (and her mother) are "Nightcomers", which is another name for a witch but specifically refers to a witch who has devoted herself to the devil, gaining additional powers:

Nightcomers are women who were once daywalkers, but have since become devoted servants of the Devil. They have the appearance of bald human beings with pale skin, and possess a glowing shade of yellow within the irises of their eyes, which are surrounded by black scleras rather than the average white. Their bodies are covered in a variety of scratches and scars that, according to Miss Ives, are the Devil's mark upon them as significance of their servitude. They are fluent in the Verbis Diablo, "The Word of the Devil", described in theology as the corruption of the angelic language.

Outside of the wiki, season 2, episode 3 is titled "Nightcomers" and it introduces the cutwife and some of Vanessa's backstory, along with her history with Poole:

Poole and her Nightcomers already have their eye on Vanessa, the Master’s favorite, but as they are unable to pass the spell-protected cottage entrance, they require human intervention. [. . .]

We now know there’s a personal component to the struggle between Vanessa and the Nightcomers, even as Vanessa herself doesn’t realize the extent of it. [. . .]

The Penny Dreadful Wiki has other useful information on the various pages about the characters, which you may find interesting.

I'd also like to point out that her name, Hecate, is likely in reference to the same goddess who appears as a character in Macbeth, responding to the calls of the three witches, and mentioned in Macbeth's famous Dagger soliloquy:

[. . .]
Which was not so before. There's no such thing:
It is the bloody business which informs
Thus to mine eyes. Now o'er the one halfworld
Nature seems dead, and wicked dreams abuse
The curtain'd sleep; witchcraft celebrates
Pale Hecate's offerings, and wither'd murder,
Alarum'd by his sentinel, the wolf,
Whose howl's his watch, thus with his stealthy pace.
[. . .]

Hecate is often considered one of the primary goddesses in Wicca:

Hecate is also one of the "patron" goddesses of many Wiccans, who in some traditions identify her with the Triple Goddess' aspect of the "Crone". In other circles Wiccan witches associate her with the "Maiden", or the "Mother" aspects as well, for Hecate has three faces, or phases. Her role as a tripartite goddess, which many modern-day Wiccans associate with the concept of "the Maiden, the Mother and the Crone", was made popular in modern times by writers such as Robert Graves in The White Goddess, and many others, such as the 20th century occultist and author, Aleister Crowley. Historical depictions and descriptions show her facing in three different directions, a clear and precise reference to the tripartite nature of this ancient Goddess; the later Greek Magical Papyri sometimes refer to her as also having the heads of animals, and this can be seen as a reference to her aspect of Motherhood; in this portrayal she is known as "Mistress of Animals". Modern Hellenic polytheists honor Hecate during the Deipnon.